Most mobile telephones now incorporate a camera unit. The camera unit typically includes a solid state imaging sensor and a lens. The camera unit must be physically small and light-weight. In addition, battery capacity is a significant problem in these and similar mobile devices.
In conventional camera units, the focus of the lens is set by mounting the lens in a support structure commonly referred to as a barrel. For autofocus systems, the barrel, and thus the lens, is moved relative to the solid state imaging sensor. This is achieved by coupling the barrel to a carriage which is driven by some form of motor.
Known forms of autofocus assemblies are relatively large and heavy, and consume relatively large amounts of power. It is also difficult to provide an accurate end stop to the lens travel corresponding to the optimum infinity setting.
Conventional drive assemblies often include coil spring arrangements with the motor to drive the lens assembly and retain it at a desired position. However, coil springs require a significant amount of space in the direction of the spring axis, since the spring must be of sufficient length to provide the travel required. Tension springs require even greater space in the direction of the spring axis to accommodate spring extension. They also provide a relatively low resistance to rotation in either direction about the spring axis. Also, many coil springs are unidirectional, unless substantially modified.
Furthermore, conventional drive assemblies typically consume relatively large amounts of power. One reason for this is that coil spring arrangements are typically relatively stiff and so they require relatively large tensile or compressive forces to displace the springs from their neutral state. Another reason is that conventional motors themselves consume relatively large amounts of power.
It is desirable to improve each of the size, weight, positioning accuracy and simplicity (particularly in terms of the number of parts) of conventional drive assemblies in camera units.
EP 05270103.4 describes a focusing system for the lens of a camera unit. The system includes a master post fixed to a frame and the barrel is slidably mounted on the post via bearings. A piezoelectric motor drives the barrel bidirectionally along the axis of the post.
Piezoelectric motors are known to be smaller, lighter, more accurate and involve fewer parts than other forms of drive motors. A known piezoelectric motor is described in U.S. Pat. No. 6,768,245 to Mock et al. and is available from Miniswys SA of Biel (CH). A disadvantage is that they typically lack available power for driving components. However, the system of EP 05270103.4 significantly reduces the friction present and so reduces the power consumed when driving the barrel. This allows the use of a piezoelectric motor. However, friction cannot be totally eliminated with this system since it will be present between the barrel and the bearings. Also, the axis of the post is necessarily offset from the barrel axis. This creates a moment about the post axis which increases the friction present.